Hydraulic fracturing has been widely used for stimulating the production of crude oil and natural gas from wells completed in reservoirs of low permeability. Methods employed normally require the injection of a fracturing fluid containing suspended propping agents into a well at a rate sufficient to open a fracture in the exposed formation. Continued pumping of fluid into the well at a high rate extends the fracture and leads to the build up of a bed of propping agent particles between the fracture wells. These particles prevent complete closure of the fracture as the fluid subsequently leaks off into the adjacent formations and results in a permeable channel extending from the well bore into the formations. The conductivity of this channel depends upon the fracture dimensions, the size of the propping agent particles, the particle spacing and the confining pressures.
The fluids used in hydraulic fracturing operations must have fluid loss values sufficiently low to permit build up and maintenance of the required pressures at reasonable injection rates. This normally requires that such fluids either have adequate viscosities or other fluid loss control properties which will reduce leak-off from the fracture into the pores of the formation.
Fracturing of low permeability reservoirs has always presented the problem of fluid compatibility with the formation core and formation fluids, particularly in gas wells. For example, many formations contain clays which swell when contacted by aqueous fluids causing restricted permeability, and it is not uncommon to see reduced flow through gas well cores tested with various oils.
Another problem encountered in fracturing operations is the difficulty of total recovery of the fracturing fluid. Fluids left in the reservoir rock as immobile residual fluids impede the flow of reservoir gas or fluids to the extent that the benefit of fracturing is decreased or eliminated. The removal of the fracturing fluid may require the expenditure of a large amount of energy and time, consequently the reduction or elimination of the problem of fluid recovery and residue removal is highly desirable.
In attempting to overcome the fluid loss problems, gelled fluids prepared with water, diesel, methyl alcohol and similar low viscosity liquids have been useful. Such fluids have apparent viscosities high enough to support the proppant materials without settling and also high enough to prevent excessive leak-off during injection. The gelling agents also promote laminar flow under conditions where turbulent flow would otherwise take place and hence in some cases, the pressure losses due to fluid friction may be lower than those obtained with low viscosity-base fluids containing no additives. Certain water-soluble, poly-acrylamides, oil soluble poly-isobutylene and other polymers which have little effect on viscosity when used in low concentration can be added to the ungelled fluid to achieve good friction reduction.
In attempting to overcome the problem of fluid compatibility when aqueous fracturing fluids are used, chemical additives have been used such as salt or chemicals for pH control. Salts such as NaCl, KCl or CaCl.sub.2 have been widely used in aqueous systems to reduce potential damage when fracturing water sensitive formations. Where hydrocarbons are used, light products such as gelled condensate have seen a wide degree of success, but are restricted in use due to the nature of certain low permeability reservoirs.
Low density gases such as CO.sub.2 or N.sub.2 have been used in attempting to overcome the problem of removing the fracturing liquid. The low density gases are added at a calculated ratio which promotes fluid flow subsequent to fracturing. This back flow of load fluids is usually due to reservoir pressure alone without mechanical aid from the surface because of the reduction of hydrostatic head caused by gasifying the fluid.
Moreover, low density liquified gases have themselves been used as fracturing fluids. Reference is made to Canadian Patents 687,938 and 745,453 to Peterson who discloses a method and apparatus for fracturing underground earth formations using liquid CO.sub.2. Peterson recognized the advantages of liquid CO.sub.2 as a means to avoid time consuming and expensive procedures involved in the recovery of more conventional fracturing fluids. Peterson however does not disclose the use of entrained proppants in conjunction with liquid CO.sub.2. The combination of a liquid CO.sub.2 fracturing fluid and propping agents has been described by Bullen in Canadian Patent 932,655 wherein there is described a method of entraining proppants in a gelled fluid, typically a gelled methanol, which is mixed with liquid carbon dioxide and injected into low permeability formations. The liquid carbon dioxide is allowed to volatize and bleed off and the residual liquid, primarily methyl alcohol, is in part dissolved by formation hydrocarbons and allowed to return to the surface as vapor, the balance, however, being recovered as a liquid using known recovery techniques. Clearly, it has been demonstrated that the need to use a gelled carrier fluid has resulted in the negation of some of the fluid recovery advantages attendant upon the use of liquified gas fracturing fluids.
Subsequent disclosures have been primarily concerned with the development of more advantageous gelled fluids to entrain proppants for subsequent or simultaneous blending with the liquified carbon dioxide fracturing fluid. Reference is made to Canadian Patents 1,000,483 (reissued as Canadian Patent 1,034,363), 1,043,091, 1,197,977, 1,241,826 and 1,242,389 in this regard. Each of these patents teaches the nature and composition of gelled or ungelled carrier fluids, typically methanol or water based, which, when blended with liquid CO.sub.2, produce a two-phase liquid system which allegedly is useful in attempting to overcome the problems of fluid compatibility with formation fluids while at the same time being capable of transporting increased concentrations of proppant material into the fracture zones.
From the foregoing, it will be readily appreciated that the use of liquid CO.sub.2 as a fracturing agent is known. It is further known to use other liquids having propping agents entrained therein for blending with the liquified gas fracturing fluid. The propping agents are subsequently deposited in the liquid or foam-formed fractures for the purpose of maintaining flow passages upon rebound of the fracture zone. It is further known that proppant materials can be introduced into a liquid carbon dioxide system if a gelled liquid, usually alcohol or water-based, is mixed with the CO.sub.2 to impart sufficient viscosity to the mixture to support proppant particles. Typically, although such mixtures can initially be characterized by a high liquid CO.sub.2 ratio, that is, the ratio of CO.sub.2 volume to the conventional frac fluid in the two phase system is high, incremental increases in proppant concentrations as the fracturing process progresses results in CO.sub.2 displacement, causing substantial declines in liquid CO.sub.2 volumes. Large residual liquid fractions must then be recovered from the fracture zones and risks of contamination increase substantially. Declining liquid CO.sub.2 ratios also mean reduced fracture conductivity.
In Canadian Patent 1,134,258 belonging to the assignee herein, it has been recognized that proppant materials can be introduced directly into a liquid carbon dioxide stream using little or no other viscosifying liquid components while still transporting significant quantities of up to 800 kg/m.sup.3 (and more in some situations) of proppant material into the fracture zones. This has been achieved by pressurizing and cooling the proppants to substantially the storage pressure and temperature of the liquified CO.sub.2 prior to blending of the two for injection down the well bore.